Recently, as Wavelength Division Multiplexing (WDM) optical transmission techniques come into practical use, the transmission capacity of optical transmission systems has rapidly enhanced up to over 1 Tb/s.
In order to operate and maintain reliably the optical transmission systems of such an ultra-large capacity, it is very important to accurately monitor Optical Signal-to-Noise Ratio (OSNR) of transmitted signal.
OSNR is defined as the ratio of signal power to noise power contained over the signal band. So transmission performance of an optical transmission system can be represented by OSNR.
In WDM optical communication networks, since each channel comes through different paths, it has different OSNR each other.
Therefore, it is not accurate to linearly predict noise intensity over the signal band from the intensity of amplified spontaneous emission (ASE) outside the signal band as proposed in the past. [Refer to H. Suzuki and N. Takachio, “Optical signal quality monitor built into WDM linear repeaters using semiconductor arrayed waveguide grating filter monolithically integrated with eight photodiodes,” Electronics Letter, Vol. 35, pp. 836–837, 1999.]
Recently, there is proposed a method where each channel's OSNR is measured using polarization characteristics of optical signal and ASE. [Refer to D. K. Jung, C. H. Kim, and Y. C. Chung, “OSNR monitoring technique using polarization-nulling method,” OFC 2000, Baltimore, March 2000, Paper WK4.]
However, the above method is limited in its application scope due to the phenomenal fact that signal's degree of polarization (DOP) is lowered after transmission because of polarization mode dispersion (PMD) and nonlinear birefringence. [Refer to J. H. Lee, D. K. Jung, C. H. Kim, and Y. C. Chung, “OSNR monitoring technique using polarization-nulling method,” IEEE Photon. Technol. Lett., vol. 12, no. 1, pp 88–90, 2001.]